A cable television network is a system for providing television services to consumers via radio frequency signals transmitted to television sets through fixed coaxial cables as opposed to an over-the-air method used in traditional television broadcasting, in which a television antenna was required. The abbreviation CATV is often used to mean “Cable TV”. It originally stood for “Community Antenna Television”, from the cable television's origins in 1948: in areas where the over-the-air reception was limited by mountainous terrain, large “community antennas” were constructed, and a cable was run from the community antennas to individual homes.
A CATV network consists of a controlling center, or so called “head end” facility, whose function is to control video and data traffic in the network by generating and, or distributing video and data signals, and a cable plant, whose function is to broadcast these signals to subscribers. Therefore, the cable plant is the communications medium of a CATV network. Typically, the cable plant comprises a broadband transmission cable over which signals are transmitted from the head end facility to home transceivers, by generating outbound or downstream signals, as well as from home transceivers to the head end facility, by generating inbound or upstream signals. Broadband coaxial cables are advantageously used in this application because, inter alia, they support a large frequency bandwidth, of about 1 GHz or more, and provide signal shielding at a moderate cost in comparison to other media. The wide frequency bandwidth permits the definition of a substantial number of channels on the cable thus allowing simultaneous transmission of inbound and outbound signals. Each signal occupies a particular frequency band, or a channel, on the cable.
CATV systems have, in recent years, moved beyond merely providing analog television signals by broadcasting these signals over the cable to subscribers in their homes. Digital video services have become more popular than analog television services due to more efficient bandwidth utilization, and due to their intrinsic high-definition video (HDTV) capabilities. Further, a subscriber of a CATV network has been given a voice in many systems. An advantaged subscriber nowadays has a transceiver, or a modem, which allows the transmission of signals upstream to the head end of the network. Among the many services that the subscribers have access to by having the transceiver or the modem are the purchase of extra-cost special programs and home shopping on television catalogue services, an Internet service, and a voice-over-IP (VoIP) phone service.
A subscriber sends an upstream signal to the head end through a TV set, a computer, a VoIP phone, or other generating means connected to his transceiver or modem. His entry is encoded in a digital format and becomes, potentially with the address of his station and other enabling data, a data packet for transmission to the head end. The data packet is used to modulate a radio frequency carrier wave and is transmitted to the head end through the cable plant of the CATV network.
In general, a given channel can effectively support only one signal at a time. If more than one signal appears simultaneously on a channel, one signal source will usually dominate the detectors tuned to the channel, excluding all weaker signal sources while it is transmitting, or, where the signal strength of individual transmitters is more or less equal, the signals will interfere with one another and the intelligence comprehended by each signal will be confused with that of the other signal. In either case, access by some or all transceivers is lost. If channel availability on a coaxial cable was unlimited, such matters would be of no consequence, because each subscriber could be given a dedicated channel on the cable for access to the head end facility. Unfortunately, this is not the case, and the number of upstream channels available is almost certain to be far less than the number of subscribers desiring to use the system.
However, the transmission of subscriber-initiated inbound signals is either of a short duration, or it can be broken into short-duration bursts. For example, an outgoing voice signal from a VoIP phone user is broken into short-duration segments, by using a real-time signal compression at the transmitter end, and is recombined using a real-time signal de-compression at the receiver end, allowing one to preserve the continuity of the voice at the receiver end. Accordingly, the system designer only needs to assign one or, at most, a handful of channels to accommodate all subscriber-generated inbound signals. As a result, a single upstream channel may potentially be used for thousands of subscribers without the interference between the signals becoming apparent to the subscriber.
The above mentioned growth of functionality of cable based networks, with more and more features and functions added to the subscriber's benefit, has to be matched by a growing effort to assure the quality of the existing services. To avoid interruption or impairment of the existing services, a periodic maintenance and testing of the network is required. Of such a testing, a frequency sweep of the network, allowing one to measure the frequency response of a CATV network between two points of the network, is a common task.
Traditionally, the frequency sweep is performed by injecting a test signal having a constant amplitude and continuously changing frequency at one point of the network, and measuring the amplitude of the test signal at another point of the network. The ratio of the amplitudes of the test signal between the two points, measured as a function of the test signal frequency, allows one to evaluate the attenuation of a signal, as a function of frequency, between the measurement and the injection points. In other words, the attenuation of a broadcasted or a subscriber-generated signal occupying a particular frequency channel or band can be evaluated from the frequency sweep test. When the attenuation rises above a certain threshold value, a reliable data reception and transmission cannot be guaranteed. A periodically performed frequency sweep test should allow the service provider to correct the problem that caused the attenuation to rise before a subscriber is aware of the problem, thus maintaining a high quality of service.
A significant drawback of performing periodic frequency sweeps, or even a single frequency sweep for that matter, is that the test signal of the frequency sweep interferes with broadcasted or subscriber-generated signals, disrupting normal performance of a CATV network. One can, of course, use only the unallocated frequencies for the sweep, and interpolate between the measured points corresponding to attenuation at those unallocated frequencies; however, as CATV networks have increased the number of channels broadcasted, the availability of spectrum to inject these test signal has decreased, resulting in problems in maintaining CATV networks and measuring the frequency response thereof.
The problem of interference during frequency sweeps has long been recognized. For example, U.S. Pat. No. 4,408,227 incorporated herein by reference, entitled “Method and Apparatus for Television Distribution System Sweep Testing”, and issued on Oct. 4, 1983 in the name of Bradley, discloses a distribution system sweep test, in which the test signal is time multiplexed with the program signal to prevent interference. The test signal is injected during vertical blanking intervals of an analog TV raster signal. The test signal locations are normally between lines 17 and 20 of an analog TV signal since the Federal Communications Commission of the United States of America has authorized test signal injection for this particular area of an analog TV signal. Similarly, U.S. Pat. No. 4,700,222 incorporated herein by reference, entitled “Apparatus and Method of Testing the Frequency Response of a Cable Television System”, and issued on Oct. 13, 1987 in the name of Large et al., relates to testing the frequency response of a CATV system by transmitting test signals of varying frequency during blanking intervals of an analog TV signal.
Disadvantageously, the systems of Bradley and Large rely on presence of blanking intervals in an analog TV signal. Since the blanking intervals are absent in digital signals, these systems cannot be used to test a modern CATV network having digital TV channels or channels allocated for an upstream traffic such as an upstream Internet traffic.
U.S. Pat. No. 5,233,418 incorporated herein by reference, entitled “CATV Sweep System Using a Gated Receiver”, and issued on Aug. 3, 1993 in the name of Gumm et al., relates to a method of determining the frequency characteristics of a CATV by inserting test pulses in the vertical interval between equalizer pulses of an RF video signal. Again, this system is not readily compatible with the abovementioned modem digital CATV networks.
U.S. Pat. No. 5,867,206 incorporated herein by reference, entitled “CATV Frequency Sweep Testing Using RF Transmitter to Generate Test Signals”, and issued on Feb. 2, 1999 to Voght et al., relates to frequency sweep testing of a CATV system by testing only unused channel frequencies. Disadvantageously, as the number of unused channels in a network decreases due to adding new services and functions as noted above, so does an opportunity of frequency sweeping a CATV network using the method of Voght.
Further, U.S. Pat. No. 6,961,370 incorporated herein by reference, entitled “Sweep Method Using Digital Signals”, and issued Nov. 1, 2005 in the name of Chappell, relates to determining the frequency response of a CATV system by obtaining an absolute signal strength measurement and a relative frequency response measurement for a selected digital channel frequency. While the method of Chappell solves the problem of obtaining the frequency response without disrupting transmission of signals on the forward path, that is, from the head end towards a subscriber of a CATV network, this method cannot be used on the reverse path, since it is the head end that broadcasts signals at various frequencies, which can be used for the frequency response measurement on the forward path of the broadcasted signal.
Finally, U.S. Pat. No. 6,278,485 incorporated herein by reference, entitled “Preconfigured CATV Sweep Testing Method and Apparatus”, and issued on Aug. 21, 2001 in the name of Franchville et al., discloses sweep testing a reverse path in a subscriber's CATV network. While the method of Franchville allows one to obtain necessary reverse frequency sweep data, it has a serious drawback of disrupting normal transmission.
An object of the present invention is to overcome the shortcomings of the prior art by providing sweep test on both the forward and the reverse path of a CATV network. Importantly and advantageously, the apparatus and the method of frequency sweeping according to the present invention allows one to perform the sweep without interfering with normal transmission of data.